The present invention generally relates to marine geophysical surveying and, more particularly, in one or more embodiments, to methods and systems for towing multiple source cables with spreader lines coupled to the source cables using open collars.
Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, may include towing an energy source at a selected depth in a body of water. In some instances, multiple energy sources, such a seismic air guns, may be towed by the vessel. One or more geophysical sensor streamers may also be towed in the water at selected depths. The streamers may be long cables having geophysical sensors disposed thereon at spaced apart locations. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with rock formations below the water bottom with changes in the energy field due to this interaction detected by geophysical sensors positioned on the sensor streamers, for instance. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
A number of different towing arrangements may be used when towing multiple energy sources from the vessel. In one arrangement, source cables may be used for towing each of the energy sources with spreader lines extending laterally between the source cables. The spreader lines are used to aid in maintaining lateral separation of the energy sources. Collars may be used to secure spreader lines to a particular source cable with the each collar being allowed to slide freely on the respective source cable. It is important for the collars to be allowed to slide freely so that spreader lines can be positioned close to the energy sources to ensure stable separation of the energy sources. However, as the towing arrangements have become more complex, additional connection points or other devices may need to be included on the source cables than can restrict or otherwise limit the ability of the collars to slide freely. If movement of the collars is limited, the spreader lines may not be able to be within sufficient proximity of the energy sources for maintaining the desired lateral separation.
Accordingly, there is a need for improved methods and systems for towing multiple cables in marine geophysical surveys, for example, that can maintain lateral separation of the energy sources.